The present invention relates to the operation, sometimes encountered in tube manufacturing, in which the radius of curvature of the bend in a U-shaped tube is reduced by squeezing, or forcing the legs toward each other but keeping them parallel. When relatively thin-walled tubes are bent, a problem is often encountered in that the bent section has a tendency to buckle or at least to have its cross section deformed to an unacceptable shape. This deformation necessitates a reshaping of the cross section of the tube after the squeezing has been completed.
Efforts to overcome this deformation are exemplified by Huet, U.S. Pat. No. 2,689,596. The Huet patent discloses complementary squeeze dies and a crotch piece. Both the squeeze dies and the crotch piece have engaging surfaces in the form of grooves whose curvature accommodates the outer diameter of the tube. In addition, the grooves are bent longitudinally to follow the desired bend in the tube. The crotch piece fits in the crotch of the U-shaped tube, while the squeeze dies fit outside and to the back of the tube. Considering the tube to be in a horizontal plane and the squeeze dies and crotch piece to be in the same plane, the Huet apparatus further includes top and bottom plates that confine the tube in a vertical direction. With this apparatus it is possible to squeeze a tube whose wall thickness is only 10% of the tube diameter. Below 10%, however, the results are unsatisfactory.